The present invention relates to a light-emitting element having a function of controlling the phase of light.
A tunable light source capable of oscillating a plurality of wavelengths by a single light source is very useful for a WDM (Wavelength Division Multiplexing) transmission system because it facilitates system reconstruction and reduces the stock of backup light sources, and research and development thereof is flourish in the recent years. Tunable light sources of this type are disclosed in, e.g., reference 1 (Japanese Patent Laid-Open No. 2-165116), reference 2 (Japanese Patent Publication No. 07-082131), and reference 3 (Hiroyuki Yamazaki et al, IEICE Electronic Society Conference 2005, Proceedings No. C-3-89).
A conventional tunable light source will be described with reference to FIGS. 11 and 12. The tunable light source comprises a semiconductor optical amplifier (SOA) 101 with a phase control region, and a ring resonator type wavelength filter 102. The ring resonator type wavelength filter 102 includes a plurality of ring resonators 103 with slightly different optical path lengths, and heaters 104 to control the temperature of the ring resonators 103.
In this tunable light source, the ring resonator type wavelength filter 102 serves as an optical circuit having a function of returning, of light entering from the SOA 101 with a phase control region, only a light component having a specific wavelength to the SOA 101 with a phase control region. The heaters 104 can control the “specific wavelength”. The ring resonator type wavelength filter 102 is described in detail in, e.g., reference 3, and a detailed description thereof will not be repeated here.
The SOA 101 with a phase control region has a gain region 107 and a phase control region 108, as shown in FIG. 12. The gain region 107 has an active layer 112 and obtains a gain for oscillation by injecting a current in the active layer 112. The phase control region 108 changes the refractive index by injecting a current in a core layer 113, thereby controlling the phase of light so as to obtain the best oscillation characteristic. The core layer 113 in the phase control region 108 is made of a semiconductor layer whose composition (shorter-wavelength composition) is different from that of the active layer 112 in the gain region 107 so that the loss with respect to the oscillation wavelength becomes small. The active layer 112 and core layer 113 are connected by a so-called butt joint 114 formed by an etching process and regrowth process. The SOA 101 with a phase control region is formed by stacking a lower cladding layer 115, the above-described active layer 112 and core layer 113, and an upper cladding layer 111 on a substrate 116. P-electrodes 109 and 110 are formed on the upper cladding layer 111. An n-electrode 117 is formed on the lower surface of the substrate 116.
As shown in FIG. 11, exit light from the end face on the side of the phase control region 108 enters one end of a connection waveguide 105 of the ring resonator type wavelength filter 102. Exit light from the end face on the side of the gain region 107 is extracted to the outside as the output light of the tunable light source. A high-reflection film 106 is arranged at the other end of the connection waveguide 105 of the ring resonator type wavelength filter 102.
In the conventional tunable light source, light absorption loss occurs in the core layer 113 in the phase control region 108, and the output optical power of the tunable light source lowers. Additionally, light reflection occurs at the butt joint 114 of the SOA 101 with a phase control region. When the reflected light returns to the gain region 107, the oscillation characteristic becomes unstable. Furthermore, the butt joint 114 of the SOA 101 with a phase control region reduces yield in manufacturing elements, resulting in an increase in element manufacturing cost.